The one-time pad cipher is a common key cryptosystem with which a key is shared between the transmission side and the reception side. With the one-time pad cipher, encryption is carried out using a cipher key of the same amount (the same bit count) as that of communication data. With the one-time pad cipher, a cipher key is used only once and will not be reused.
A typical example of the one-time pad cipher includes a Vernam cipher, with which bitwise exclusive OR of communication data and a cipher key is calculated and the calculated result is treated as encrypted data.
Cipher communication by means of a one-time pad cipher requires a cipher key of a large amount because it consumes the same amount of the cipher key as that of the communication data. With the one-time pad cipher, once the cipher key is exhausted, cipher communication can no longer be performed.
Patent Literature 1 includes a description on the choice of a cryptosystem to apply between a one-time pad cipher and a block cipher depending on the significance of the communication data to be encrypted. This reduces the consumption of the cipher key in the one-time pad cipher.
Patent Literature 2 includes a description on how the accumulated amount of the cipher key of each terminal device is monitored so that a cipher key is generated with a higher priority for a terminal device having a small accumulated amount. This is aimed at prevention of the exhaustion of the cipher key in a particular terminal device.